In manufacturing a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or a MISFET (Metal Insulator Semiconductor Field Effect Transistor), a method to lower an impurity concentration of a channel region is used so as to improve ON current. When the impurity concentration of the channel region is lowered, threshold voltages of the MOSFET or the MISFET decrease. For the reason, when the impurity concentration of the channel region is decreased so as to improve the ON current, a problem may arise that a threshold voltage decreases and a predetermined threshold voltage can not be obtained.
As means to solve the problem, a technique is known that a threshold voltage is increased by forming a hafnium layer, for example, between a gate insulating film and a gate electrode. A method to increase a threshold voltage using a hafnium layer is disclosed in Japanese Patent Application Publication 2006-332179. According to the method, decrease of a threshold voltage, which is caused lowering an impurity concentration of a channel region, can be compensated by increasing a surface density of a hafnium layer. As a result, it becomes possible to increase ON current of a MOSFET or a MISFET while keeping a predetermined threshold voltage.
However, it has not been known that an optimum surface density of a hafnium layer corresponding to improvement in performance of a CMOSFET (Complementary Metal Oxide Semiconductor Field Effect Transistor) or a CMISFET (Complementary Metal Insulator Semiconductor Field Effect Transistor).